The conference this year will feature a keynote address by Pete Lilly, chief operating officer of Consol Energy.

Frank Chase, a National Institute for Occupational Safety and Health researcher, will outline multiple seam mining interactions in reference to case histories from the Harris No.1 Mine in West Virginia. Harris has experienced roof falls, rib sloughage, and gateroad and bleeder entry closure.

Of 19 instances of multiple seam interactions, 71% of the most severe interactions occurred when the depth of cover was 1000ft or greater; the pillar stability factor was less than 1.50; the overburden/interburden thickness ratio was 3.9 or greater; and the upper seam stability factor was 1.00 or lower. Heavily loaded “critical” pillars may cause the most severe stress conditions in the underlying seam.

Leo Gilbride, consultant with Agapito Associates, describes the impact of horizontal stresses at West Elk.

Mines in Colorado have been found to have highly anisotropic horizontal stresses. (Anisotropic is the condition of having different properties in different directions.)

Measurements at mines have found the measured maximum horizontal stress to be three to four times higher than the minor horizontal stress.

At the West Elk Mine, maximum and minimum horizontal stresses of 3500psi and 800 psi, respectively, have been measured at depths of 2000ft, causing a number of unique ground control issues.

Under highly anisotropic stress conditions, ground control problems associated with both very high horizontal stresses and very low horizontal stresses can develop.

West Elk’s experience has lead to an improved use of ground support and safer mining operations under heaving ground conditions at depths exceeding 2000ft.

John Kucish, manager at Peabody Federal No.2 mine, will describe the use of fully grouted torque tension bolts.

The mine recently changed from an 8ft long combination bolt to a 6ft torque tension system. The combination bolt, supplied by Minova, consists of a 4ft, 0.804in diameter rolled deformation bar thread coupled to a smooth 7/8in headed bar, which is tensioned by rotation into the threaded coupler on setting of the resin. The change was initially driven by the failure of the bolt at the threaded coupler, due to lateral movement caused by horizontal stress.

A complete headgate entry has been successfully supported with this system and the mining of the adjacent longwall panel was completed in September. The torque tension system provided a 25% savings over the previously used bolt system.

Paul Krivokuca, mine manager, Quinsam, describes mining practices at the Quinsam room and pillar operation in Canada where all roof bolting materials are supplied by Jennmar.

Quinsam mines in a fully mechanized retreat room-and-pillar operation at a rate of 3100 tons per day from two development sections and one depillar section, at a shallow depth with cover averaging from 30-170m.

Five entry panels from 700-1200m in length are developed with rectangular pillars 16m x 38m or 44m in size, rib to rib. A 30m barrier pillar is left between the panels and adjacent gob. Development entries are 6m wide by 2.5m high.

Primary ground support at 3 North/2 North mine consists of 8ft long tensioned resin point-anchored bolts spaced on 4ft centers with 8in x 8in plates and wire roof mesh.

At 2 North mine, testing of 8ft full column resin forged head bolts is carried out with 6in x 6in plates and wire mesh in order to hold mudstone/siltstone rock and the rider coal seam as a beam in the immediate roof.

At 4 South mine, primary roof support consists of 6ft long tensioned resin point-anchored bolts spaced on 4ft centers with 6in x 6in plates, again five bolts in a row. Testing and research is underway to introduce 4ft or 5ft long full column resin forged head bolts on a 5ft x 5ft pattern.

Luis Giraldo, manager at UTD, outlines a new rock drilling technology – the Helical Drag Bit (HDB) that improves the pullout strength of fully grouted roof bolts.

A study was conducted to quantify the benefits of borehole conditioning and improve roof bolt performance. The HDB was used to condition the hole. It cuts a prescribed helical groove in the wall of the borehole. When a bolt is grouted into such a hole the grout fills the helical groove, providing a stronger, more reliable mechanical lock between the rock and grout.

Tests show greater load bearing capacity is achieved at mines with weak roof rock.

Efforts are now focused on the development of a bolting system using the HDB principle, working with existing bolting equipment.

Craig Compton, engineering technician (NIOSH), describes pull tests of fully grouted roof bolts related to resin.

The result showed no significant difference between the pullout loads of offset head bolts and standard bolts.

Pressures generated during bolt installation were tested, appearing to confirm Australian work that showed significant pressures could be generated during bolt installation. These pressures have been shown to lead to resin loss from bolt holes in weak roofs.

Dion Pastars, geologist, Solid Energy New Zealand, covers load transfer characteristics of gloved resin bolts.

Tests have shown performance loss when resin is not properly mixed. A gloved bolt generates 10 – 15% load transfer of a normally installed resin bolt.

For effective load transfer, adhesion of the resin plays a more important role than that of mechanical interlock.

Syd Peng, chairman and Charles T Holland professor, West Virginia University, looks at the effect of multi-seam mining on entry stability.

A West Virginia mine was mining the Sewickley Seam, bout 90ft above the Pittsburgh Seam, mined by room and pillar in the 1960s.

Roof falls and other stability anomalies of entries in the Sewickley Seam were found to be closely related to the location of barrier pillars and gas well pillars left in the Pittsburgh Seam. It was found that where pillaring was conducted in the Pittsburgh Seam the stress concentrated less in the Sewickley Seam. High stress concentrations were observed near the edge of gas well pillars and barrier pillars.

Tom Barczak, research physicist (NIOSH), looks at the efficacy of standing support in tailgates.

Understanding how much control standing support has on ground behavior is critical to optimize the use of these technologies in tailgate entries. Modeling to look at the effect of standing support on main roof and floor behavior concluded that standing supports do not have sufficient capacity to control main roof or floor loading.

This “uncontrollable convergence” must however be considered in the support design to prevent premature failure of the support.

As rock structure deteriorates, standing roof supports can have some impact on the strata response, and the capacity and stiffness of the standing support can be critical to the stability of the opening.

Further information may be obtained at: www.mine.cemr.wvu.edu/